Difference Between Dot 3 vs Dot 4 Brake Fluid

By Dan Collins

Published 13:16 pm

While there may not seem to be much of a difference between the names DOT 3 and DOT 4 brake fluid, there are some differences which you should know about. First up, you need to know the function of brake fluid in a car. Essentially, it is a hydraulic fluid which plays a central role in the braking system. Stored in the brake lines, it puts pressure on the rotors located in each corner of the vehicle.

When you press the brake pedal in your car, the force leads to pressure in the brake line, which causes the rotors to squeeze the pads. This friction causes the wheels to stop turning and the vehicle to stop moving. In turn, this turns the kinetic energy into heat energy, which is where the brake fluid comes into play.

Differences Between Dot 3 and Dot 4

Glycol-based brake fluids can be broken down into several numerical categories: 3, 4, 5, 5.1. There is no single standard brake fluid formula, but they do have to meet certain standards which the government set out. The specifications need to meet requirements relating to a range of different categories including pH values, chemical stability, water intolerance, and oxidation resistance. The most standard type of brake fluid used by regular drivers is DOT 3. It is suitable for vehicles which do not use their braking system overly aggressively. Since DOT 4 has a higher boiling point, it is more commonly found in police cars and racing vehicles. While DOT 4 is compatible with DOT 3, it does not work the other way around.

Varying Boiling Points

As we have just mentioned, the most obvious difference between DOT 3 and DOT 4 brake fluid is the boiling point, which is the temperature that the fluid evaporates. This also impacts their water absorption, but both of them are hygroscopic, which means that they do. But since it has a lower boiling point, DOT 3 tends to absorb water. Hard braking is more likely to boil. There are two types of boiling points – dry and wet. The former refers to fluid from a new container, while the latter refers to fluid which has been contaminated by 3.7% water. The second of the two is the boiling point that will occur during regular driving. Every time you take off the reservoir cap to put in fluid, the quality will degrade. You need to flush the brake system through every now and again to ensure that it stays in tip top condition.

Boiling Capacity

Following on from the previous point, we also have a difference in boiling capacity to discuss. DOT 3 will function well in both water and open air as it has both wet and dry boiling capacities. As for DOT 4, while it has a great dry boiling capacity, the same cannot be said when it is in water. Again, the former tends to be the one used in standard vehicles, but it is the latter which tends to be best in fast racing vehicles or if you take your car on rough driving expeditions, as it is better in looking after your braking components.

Chemical Structures

The chemical structures of DOT 3 and DOT 4 brake fluid is another way that they tend to vary. The former is often based on diethylene glycol (DEG). While this is not a prerequisite, it does seem to be one which brake fluid manufacturers have adopted. As for DOT 4, they tend to consist of glycol and borate ester. The latter substance enables the fluid to deal with higher temperatures. Both the dry and wet boiling points tend to be higher, and the chemical ingredients help to facilitate a high level of water tolerance and stability under high temperatures.

Cautions to Take

There are a couple of cautions to bear in mind when it comes to brake fluid. Since both varieties will cut through paint without so much as a second thought, you should be very careful not to spill them on the body of your car. Also, if they are mixed with other fluids, they can end up having a bad reaction. When it comes to storage of brake fluid, you should make sure that you keep it in tightly sealed containers. The chemical components of the fluid can get degraded by moisture in the air, so this is another factor which is worth bearing in mind. Therefore, you should avoid using fluid taken straight from an open bottle.

Conclusion

As a general rule of thumb, the main difference between DOT 3 and DOT 4 brake fluid is the difference in the boiling points of the substances. Remember, brake fluid can war out, so you need to make a point of replacing it from time to time. If you don’t, your brakes can end up becoming spongy and unsafe, and the moisture can end up corroding the metal components in your car. In standard passenger vehicles, a good rule to follow is to change the fluid every other year. But if you have a professional-standard racing vehicle, you should get into the habit of changing the brake fluid every single year. Unfortunately, brake maintenance is something which tends to get overlooked by a high percentage of car owners, so take this blog post as a reminder to do something about this essential part of your car.

When you are changing the brake fluid in your own vehicle and you are feeling unsure what is best to pump into your vehicle, you should check the specifications of the manufacturer for more information. But the most likely type of brake fluid that you will be using is DOT 3 fluid. Obviously, the differences aren’t vast, but they are enough to affect the performance of your vehicle, so you should bear this in mind the next time you are completing some basic maintenance on your vehicle.